High-frequency tunable devices such as high-frequency filters, high-frequency antennas or phase shifters incorporate a thin film capacitor, composed of an upper electrode, a lower electrode and a dielectric layer formed between the two electrodes, as a variable capacitance element (tunable element). Thin film capacitors function as capacitors that change the capacitance thereof according to a change in voltage applied between the electrodes. Dielectric thin films formed using a perovskite oxide such as strontium titanate (SrTiO3), barium strontium titanate (BST) or barium titanate (BaTiO3) are used for the dielectric layer that composes these thin film capacitors. In addition to physical vapor deposition methods such as vacuum deposition, sputtering or laser ablation, or chemical vapor deposition method (CVD), or chemical solution methods such as sol-gel methods are also used to form dielectric thin films (see, for example, Patent Document 1).
An example of one of the properties used to evaluate thin film capacitors used in such tunable devices is variability of capacitance versus applied voltage (tunability), and a thin film capacitor that is able to control capacitance over a wide range when a voltage is applied, namely a thin film capacitor having high tunability, is preferred. The reason for this is that the higher the tunability, the greater the ability of a thin film capacitor to accommodate a broader resonance frequency band in response to smaller changes in voltage. More specifically, when the capacitance prior to application of voltage is defined as C0V and the capacitance after a voltage tV has been applied is defined as CtV, then tunability is represented by (C0V−CtV)/CtV×100(%). For example, as shown in FIG. 5, although capacitance changes from C0V in the absence of an applied voltage to C5V when a voltage of 5 V is applied, tunability becomes higher the larger the range from C0V to C5V at this time, and such a thin film capacitor can be said to be a high tunability thin film capacitor. A tunable capacitor has been disclosed as an example of a technology for enhancing tunability in which high tunability is able to be ensured by using a material having a high dielectric constant while maintaining desired impedance during use in a high-frequency band (see, for example, Patent Document 2). The tunable capacitor disclosed in Patent Document 2 ensures high tunability at low capacitance by forming a second dielectric layer having a lower dielectric constant than a first dielectric layer between the first dielectric layer and a top electrode so as to cover a portion of the principal surface of the first dielectric layer.